This stems back to ancient times in the Mediterranean and in Middle East, when a karat became used as a measure of the purity of gold alloys. The purity of gold is now measured also in terms if fineness, That is, parts per thousand. Thus 18 karats is 18/24th of 1000 parts = 750 fineness.
Gold is made up of a large number of different bars of different weights. The most well known are the large 'London Good Delivery Bars' which are traded internationally. These weigh about 400 Troy Ounces, i.e. 12.5 kg/ 27 lbs Each. Others are denominated in kilogram's, grams, troy ounces, etc. In grams, bars range from 1 g up to 10 kg. In troy oz, from 1/10 tr.oz. up to 400 tr.oz.. Other bars include tola bars and Tael bars.
Diamond is the hardest natural material to be found on earth, and the simplest of all the gemstones in composition. The conditions that created diamonds have not existed on this planet for millions of years. Very early in the history of earth as solid matter became condensed into a sphere, the material at the heart of the planet became subjected to unbelievable extremes of temperature and pressure. it was the conditions in those deep layers that caused deposits of pure carbon to begin to crystallize into diamonds. A diamond consists of pure carbon, and although there is no chemical difference between carbon powder and diamond, the physical difference between carbon powder and the diamond crystal is both enormous and miraculous. As the outer layer of the earth cooled, stress developed, and the plates of solid rock shifted and split. Streams of liquid rock (magma) were forced to the surface in volcanic eruptions, and some of that material carried with it the diamond crystals that had begun to form deep within the earth.
"The 4 C's determine the quality and value of a diamond, and explain why some are rarer- and so more valuable-than others. The 4 C's relate to a diamond's: Cut,Color,Clarity,Carat Weight.
We have already considered most of the essential facts about cutting. However customers may confuse the quality and precision of the cut with its shape-oval, marquise, pear etc- appoint to bear in mind. There are many shapes and sizes of diamonds. But whatever the shape or size, the CUT is technical feature which can vary in quality, and which is unique to each stone. The better the quality of the cut, the better the stone will create Brilliance and Fire and that determine the value of each stone.
International standards have been established for grading diamonds according to extremely subtle differences in color. The main scales are those of the G.I.A (Gemological Institute of America), of I.D.C. (International Diamond Committee), and of C.I.B.J.O. (International confederation of Jewellery, Silverware, Diamonds, Pearls and stones).
The last of the 4C's is the carat weight. A diamond's weight is the simplest of its characteristic to measure, and from the earliest times it was used to calculate the value of a diamond. The carat is a unit of weight which derives from the carob seed. The pods of the carob or locust tree contain tiny seeds which are remarkably consistent in weight. These seeds were used by early gem traders to weigh their diamonds. A 1 carat diamond used to equal the weight of a carob seed, but in today's terms the carat is a standard metric weight of 0.2grams, and each carat is divided into 100 cents. So for example, a quarter of a carat is 25 cents, written as 0.25; a half a carat is 50 cents, written as 0.50, and so on.